ASR technology enables microphone-equipped computing devices to interpret speech and thereby provide an alternative to conventional human-to-computer input devices such as keyboards or keypads. For example, vehicle telecommunications devices can be equipped with voice dialing features enabled by an ASR system. The ASR system typically includes a hands-free microphone to receive speech from an occupant of a vehicle. The hands-free microphone is usually located in a forward portion of a passenger compartment of the vehicle, such as in an instrument panel, an A-pillar molding, a rear view mirror assembly, a headliner, overhead console, or the like. Such a forward-positioned microphone is generally satisfactory to enable reliable recognition of speech from a driver.
A forward-mounted microphone may be susceptible to airflow noise due to local pressure variations in an air stream such as from windshield defroster vents, open windows, or open roofs. Accordingly, some ASR systems deploy complex digital signal processing and noise cancellation techniques, or multiple microphone arrays, to reduce the influence of airflow noise. But these approaches add cost and complexity to the ASR system. Therefore, windguards are often provided to ameliorate the effects of rapidly moving air over a microphone.
Many windguards are susceptible to Helmholtz resonance, which is a phenomenon of air resonance in a cavity. When air is forced past an acoustic inlet of a windguard, the air pressure inside tends to cyclically increase and decrease, thereby causing vibration and noise that a microphone can pick up, similar to the sound created when one blows across the top of an empty bottle. Thus, such resonant sound can produce poor signal-to-noise ratios from a microphone, thereby rendering conventional windguards counterproductive.